The present invention relates to a turbomolecular pump which can attain a good pumping efficiency even in a low vacuum range. As described in "Recent State in Turbomolecular Pump Development" in the pages 25 through 32 of the Proc. 7th Intern. Vac. Congr. & 3rd Intern. Conf. Solid Surfaces (Vienna 1977), the turbomolecular pump is designed to mechanically blow gas molecules for exhaustion thereby achieving ultra-high vacuum. To this purpose, the pump body comprises rotors consisting of inclined blades rotatable at a high speed and stators consisting of blades inclined in the inverse direction to the rotors brades. And these rotors and stators are arranged alternately, usually, in multiple stages.
With this type of pump, however, since the compression ratio in the multiple stage blades is low, the pumping performance drops seriously in a low vacuum range (as the vacuum approaches atmospheric pressure). To reduce this disadvantage, a number of rotor and stator blades may be axially provided to define the pump body. However, this construction results in a heavy weight, interferring with high-speed rotation, and is disadvantageous in terms of cost. Specifically, with conventional turbomolecular pumps, the pumping speed suddenly drops at a vacuum of 10.sup.-3 .about.10.sup.-2 Torr and becomes virtually zero at a vacuum of about 0.1 Torr. When a turbomolecular pump is to be operated in a low vacuum range, therefore, it has been general to connect auxilliary vacuum pumps such as a mechanical booster pump and a rotary pump, as appropriate, to the turbomolecular pump, so that the pumping capacity is compensated.
So-called a hybrid turbomolecular pump has been also developed to reduce the necessity of these backing pumps. This type of pump comprises a group of rotor and stator blades staggered on the suction port side and a stator having spiral thread groove on the exhaust port side for guiding gas sent from said group of the blades to the atmosphere. The depth of the thread groove is formed so that is gradually becomes shallower (toward the exhaust port) to increase the compression ratio. With this hybrid pump, the pump body itself does not still provide a compression ratio raising function and requires a stator with complex thread groove, thus involving complicated machining work for manufacture.
Though the hybrid turbomolecular pump attains good pumping efficiency in a low vacuum range, it presents a new problem. Specifically, the hybrid pump is so designed that gas molecules are discharged as guided along the long passage defined by said spiral thread groove, with circumferential momentum given to the gas molecules on the rotor surface. This passage in the spiral groove has no node on the way, thus allowing gas molecules to flow in any direction. As a result, backstreaming of molecules is easily caused so that ultimate vacuum achieved may be deteriorated.